This invention relates to fuel element assemblies for nuclear reactors of the type used in pressurized water nuclear power stations. It relates particularly to a quick release top nozzle assembly for use in a fuel element assembly.
The fuel for a nuclear reactor of the type used in pressurized water nuclear power stations is a plurality of sintered cylindrical (about 0.4 inches [10.2 mm.] in diameter) uranium dioxide pellets stacked inside a long Zircalloy tube closed at both ends to form a fuel rod. Individual fuel rods, are inserted into cells of a series of spaced, parallel metal support grids joined together with thimble tubes. The grids typically arrange the fuel rods into a 14.times.14 or 17.times.17 square array, to comprise a fuel element assembly. A series of such fuel element assemblies are then arranged in parallel groups to comprise the core of the nuclear reactor.
When fuel element assemblies are in operation in a nuclear power plant, the fuel rods may become damaged and result in the escape of radioactive materials into the primary coolant in the reactor. If this occurs, the utility must remove the fuel element assembly containing the damaged fuel rod from the reactor to a separate location during the refueling outage, remove the damaged fuel rod from its particular grid cell in the fuel element assembly, insert a new fuel rod into the grid cell and then return the fuel element assembly back to its place in the reactor core. It is vital that such replacement of a damaged fuel rod be accomplished as easily and quickly as possible since fuel shuffles are usually on the critical path of the refueling outage.
Fuel element assemblies are usually provided with removable top nozzle assemblies that provide access to the individual fuel rods. A typical prior art removable top nozzle assembly is shown in FIGS. 1a and 1b, which illustrate the prior art removable top nozzle assembly unassembled in FIG. 1a, and assembled in FIG. 1b.
As shown in FIGS. 1a and 1b, the typical prior art removable top nozzle assembly 1 was comprised of a metal top adapter plate 2 having a bore 3 with a circumferential groove 4 adapted to receive a mating circumferential rib or bulge 7 on the slotted upper end of a metal tubular nozzle insert 5. The tubular nozzle insert 5 was retained in the bore 3 of the top adapter plate 2 by a ribbed metal lock tube 6 inserted into the top of the nozzle insert 5, as shown in FIG. 1b. The lower end of the nozzle insert 5 was provided with a plurality of spaced additional circumferential ribs or bulges 7 adapted to receive mating ribs or bulges 8 on a metal thimble tube 9 that fit into the lower end of the nozzle insert 5 and into an opening in the top grid support member 10 of the fuel assembly.
The typical prior art top nozzle assembly 1 illustrated in FIGS. 1a and 1b, presented several problems to the operator when replacing the fuel rods. First, removal of the top nozzle assembly 1 to permit the removal and replacement of the fuel rod was often difficult because of the force necessary to separate the adaptor plate 2 from the tubular nozzle inserts 5 was often excessive. Second, once the adaptor plate 2 was removed, the protruding tubular nozzle inserts 5 are left exposed and subject to possible damage during the fuel rod replacement operation. Third, the prior art top nozzle assembly designs required very careful alignment of the assembly components during reassembly and usually required special alignment tooling and procedures which often extended the time needed to replace a damaged fuel rod. Fourth, the prior art top nozzle assembly designs required excessive handling of the individual components making up the top nozzle assembly 1. For example, each lock tube 6 had to be removed individually from its nozzle insert 5 and carefully examined for possible damage prior to its replacement in the top nozzle assembly 1.